1. Technical Field
The invention relates generally to an apparatus and method for determining fluid flow patterns. More particularly, the invention relates to an apparatus and method for detecting anomalies in a flowing stream.
2. Background Art
Measurement of fluid flow is an integral part of many industrial processes, and it is necessary to obtain accurate measurements of fluid flow because a small error in measurements will significantly impact control and operating cost of the processes. The rate of flow or quantity of a moving fluid in a closed conduit is typically measured by a flowmeter, e.g., orifice meter, turbine meter, ultrasonic meter, or other devices that respond to flow profile, i.e., velocity distribution in a cross-sectional area of flow. Generally, the accuracy and repeatability of measurements obtained from the flowmeter are influenced by the condition of the measured flow. For best results, the measured fluid must flow through the primary measurement element, e.g., orifice plate in the case of an orifice meter, free of anomalies such as swirl, jetting, irregular flow profile, or other irregular flowing conditions that are unusual, abnormal, non-conforming, or undesirable and which do not meet requirements stipulated by industrial standards or prescribed rules of practice.
FIGS. 1A and 1B illustrate a regular flow profile 10 for fluid flow in a circular pipe 12. As shown, the regular flow profile 10 is parabolic in shape and symmetric about the longitudinal axis 13 of the pipe. Any flow that deviates from this symmetric, parabolic flow profile may be considered to be irregular. FIG. 1C illustrates an example of an irregular flow profile 14 for fluid flow in a circular pipe 16. In a piping system, an irregular flow profile, swirl, or other irregular flowing condition may be the result of upstream pipe lengths, offsets, ridges, bends, elbows, changes in pipe size, and other piping conditions. FIGS. 1D-1H illustrate examples of anomalous flow patterns for various piping conditions in a piping system. Flow profile and swirl may also be induced by valves, flow controllers, pressure reduction equipment, and flow restrictions. Excessive internal pipe wall roughness, irregular or uneven surfaces caused by rust, scale, welding irregularities, fabrication processes, manufacturing imperfections, or pipe wall irregularities may also create irregular or unexpected flow conditions in a pipe, conduit, or other type of equipment.
The exact amount of swirl or flow profile distortion adversely affecting meter performance depends on many variables and is not completely defined. Generally, if the measured flow is a Newtonian fluid, e.g., all gases, most liquids, and most dense-phase fluids associated with the petroleum, petrochemical, natural gas, and other industries, flow profile or swirl can be brought to acceptable limits by providing sufficient straight pipe upstream of the flowmeter. What is considered to be sufficient straight pipe upstream of the flowmeter depends on the type of flowmeter and whether the measured fluid is a liquid or gas. Various standards from American National Standards Institute, International Standards Organization, American Petroleum Institute, American Gas Association, and other organizations have been established to provide practical guidance for determining sufficient upstream pipe length. For example, the American National Standards Institute orifice metering standard ANSI/API 2530 provides detailed information regarding piping requirements upstream and downstream of an orifice plate.
When it is impractical to provide sufficient straight pipe upstream of the flowmeter, flow conditioners are installed at a sufficient distance upstream of the flowmeter to help eliminate or prevent irregular flowing conditions. FIGS. 2A and 2B show examples of flow conditioners that are commonly used to eliminate swirl in a flow. In many instances, these flow conditioners will have little effect on irregular flow profile. However, there are other types of flow conditioners that can eliminate swirl as well as irregular flow profile. Typically, flow conditioners are employed to redirect fluid flow patterns to produce flow profiles that are favorable to the performance of a particular flowmeter or other device when anomalies are present in the flowing stream. In the absence of anomalies in the flowing stream, flow conditioners maintain the current flow pattern and prevent development of flow pattern problems. Again, there are various industrial standards that provide detailed information regarding fabrication and use of various types of flow conditioners.
Flow conditioners can, however, be very expensive. In addition to being expensive, flow conditioners cause additional pressure drop, which results in additional costs because devices, such as compressors and pumps, must work harder and consume more energy to move the same amount of fluid. Therefore, it would be useful to be able to determine if anomalies are present in a flow before purchasing and installing flow conditioners in the piping system. It would also be useful to be able to determine the type and severity of each anomaly, if any, in the flow. By knowing the type and severity of the anomaly in the flow, the appropriate flow conditioner for the detected anomaly can be installed.
There are various methods for detecting anomalies in a flowing stream. One method uses pitot tubes to measure local static and stagnation pressures at various locations in the flow. From the pressure measurements, local flow velocities are then calculated. These local flow velocities may then be used to construct the flow profile, which may be compared to what is considered to be a normal flow profile to determine if there is an anomaly in the measured flow.
Another method for detecting anomalies in a flow involves the use of laser beams. A laser beam is focused on a small-volume element in the flow through an optical lens. Other lenses are positioned to receive the laser beam that is transmitted through the fluid and some portions of the laser beam that get scattered. When a moving particle in the small-volume element scatters the light, the scattered light experiences a Doppler shift in frequency that is directly proportional to the flow velocity. A flow profile may then be obtained by focusing laser beams on a series of small-volume elements in the flowing stream. Again the flow profile may be compared to a normal profile to determine the presence of an anomaly.
Another method for detecting anomalies in a flow involves injecting a marker, such as a colored dye or smoke, in the flow stream. As illustrated in FIG. 3, a marker is injected from a marker source 30 through an opening 32 in a pipe 34. As the fluid moves through the pipe, the marker traces streamlines and allows visual detecting of anomalies, such as swirl or jetting. This method is primarily of qualitative utility in that direct fluid measurements, e.g., flow velocity, are difficult to obtain.